(1) Field of the Invention
The present invention relates to low-cost (meth)acrylic ester binders and such binder comprising compositions, the synthesis of such binders starting from polyesters such as polyethylene terephthalate (PET), and their application in reactive acrylic systems, particularly in adhesives, coatings and floorings.
(2) Description of the Related Art
The use of polyesters, in particular PET, in film and fiber applications has increased drastically in the past several years. According to the Sherwin-Williams patent U.S. Pat. No. 5,252,615, PET accounted for about 20% by volume of the world""s solid waste in 1993. A large amount of scrap and waste materials is still resulting from the production and consumption of PET every year and therefore, processes which can make use of recycled PET are economically interesting. Taking into account that environmental care and ecological balance have a high priority for the chemical industry today, the conversion of PET waste and scraps into usable raw materials is a continuous challenge. Besides reducing waste, the use of recycled plastics contributes also to the ecological compatibility of commercial products.
Several methods have been developed for converting PET into aromatic polyester polyols, which are valuable, and low-cost raw materials for the production of adhesive, coating and flooring compositions.
Adhesives, coatings and floorings have to fulfill several requirements in order to be considered for industrial applications. Preferably, the compositions should combine a long open time with a fast curing speed. The cured systems should exhibit good mechanical and adhesive properties, besides high weather and chemical resistance. However, not only the technical requirements but also the economical aspects determine the choice of adhesives, coatings and floorings for industrial applications. The price of a composition depends mainly on the costs of its reactive components, therefore, it is important to reduce the costs of said reactive components as much as possible. Processes, which make use of inexpensive starting materials such as PET, allow the production of low-cost components for adhesives, coatings and flooring compositions.
For industrial applications of such compositions, high speed in curing, especially at room temperature, is advantageous, particularly in manufacturing operations where it is expensive to wait until the systems are fully cured. Consequently, compositions with a high curing speed and enough long open time to allow application are desired. One class of systems, which can be adapted more readily to fit these requirements, is the class of polymerizable acrylate based systems. Acrylic compositions that cure according to a free radical polymerization mechanism reach final strength within minutes at room temperature. This fast strength build-up confers acrylic compositions advantages in comparison to compositions based on epoxies and urethanes which need several hours to reach the same grade of strength.
Methyl methacrylate (MMA) is the most widely used monomer in radically curing acrylic systems. Despite the low cost and excellent properties of this monomer, its strong and disagreeable smell restricts to a great extent its use in adhesive, coating and flooring compositions. All other commercially available low-odor acrylic monomers are expensive, and for that reason novel low-odor/low-cost acrylic monomers, which can be used for acrylic systems, are needed.
Next to the fast curing, acrylic systems are also known for other advantages like high strength and high chemical resistance to acids and alkalines. A disadvantage of the standard acrylic systems is the relatively high shrinkage. Large shrinkage on curing leaves substantial residual stress, which can lead to delamination of a joint. The shrinkage is correlated to the molar mass of the used monomers. For high molar mass oligomers, the relation between the molar mass and the amount of reactive groups is higher, therefore, the shrinkage will be lower. U.S. Pat. No. 3,194,783 describes formulations with high mechanical properties and low shrinkage based on epoxy-acrylates. These oligomers are widely used in compositions for adhesives or floors. But the diacrylate of bisphenol-A diglycidylether is a very high viscous compound, which has to be diluted. The acrylates derived from ethoxylated bisphenol-A diglycidylether have a lower viscosity, but with increasing ethoxylation grade the tensile strength decreases. Moreover, the ethoxylation process is not a simple reaction procedure.
Acrylic compositions, containing cycloaliphatic compounds, with high strength and low shrinkage are described in DE 3 940 138 A1. These cycloaliphatic compounds are unsaturated esters of glycol monodicyclopentenyl ethers and have a low viscosity. But the reaction of dicyclopentadiene with glycols to form this intermediate is very difficult and time consuming.
PU-acrylates, polyurethane prepolymers, which are capped with ethylenically unsaturated end groups, are frequently used in acrylic systems and have a low shrinkage. The moderate-to-long chain prepolymers employed, comprise polyether-urethane or polyester-urethane derivatives formed by reacting a polyether or a polyester polyol with a diisocyanate. Polyether based PU-acrylates have low mechanical properties and temperature resistance. Polyester based PU-acrylates have a high viscosity or are crystalline at room temperature, and show poor compatibility to acrylic monomers.
It is also already known that acrylic ester monomers can be synthesized by direct esterification of (meth)acrylic acid with alcohols in the presence of an acid catalyst, a polymerization inhibitor, and an inert solvent to azeotrope the water formed during reaction. Several syntheses that use polyols as the alcohol component have also been reported. In general, all described processes for the synthesis of acrylic ester monomers from (meth)acrylic acid have the following limitations: 1. use of high cost raw materials and 2. production processes requiring expensive and time consuming reaction work up and product purification.
U.S. Pat. No. 3,645,984 reports the preparation of acrylic ester monomers by reacting a diol with (meth)acrylic acid or their anhydrides or acid chlorides. In EP 0 519 410 A2 aromatic polyether alcohols, prepared from aromatic alcohols and ethylene oxide or propylene oxide, are reacted with (meth)acrylic acid to yield novel acrylic ester monomers. In both patent documents, after the esterification has finished, the unreacted (meth)acrylic acid and catalyst are neutralized and the mixtures are washed several times with aqueous solutions. Then, the inert solvent is removed and the acrylic ester monomers are purified by extraction or distillation to separate them from side products and polymerization inhibitor. Besides being time consuming, the washing steps and (meth)acrylate purification cause considerable product loss decreasing the yield of the process.
EP 0 126 341 A2 and EP 0 921 168 A1 report processes for the synthesis of (meth)acrylic ester monomers by direct esterification of (meth)acrylic acid with polyester and/or polyether alcohols and polyols, and subsequent reaction with epoxies. According to these documents, the washing steps and purification of end products are avoided by reacting with epoxies the (meth)acrylic acid remaining in the mixture after esterification. For this purpose, after the esterification has finished, the inert solvent is removed and the esterification catalyst is neutralized. Then, an epoxy amount corresponding to the equivalent acid content of the mixture is added and the reaction is carried out at about 100xc2x0 C. in the presence of a suitable catalyst. Though the end products can be obtained in high yields and can be further used without purification, the reaction with epoxies implies an additional process step. The high temperatures needed for the reaction could also lead to polymerization of the synthesized (meth)acrylates. For the reaction with epoxies, the reaction mixtures must contain a certain amount of (meth)acrylic acid, and, therefore, the direct esterification has to be achieved in the presence of (meth)acrylic acid excess, or additional acid has to be added after esterification.
It is furthermore already known that PET can be used as starting material for the production of polyester polyols useful in reactive polyurethanes compositions. E.g. U.S. Pat. No. 4,048,104 relates to polyisocyanate prepolymers, prepared from PET digestion products, which are usable in polyurethane elastomers, adhesives and foams. PU-foams with increased physical strength properties are prepared in U.S. Pat. No. 4,223,068 and U.S. Pat. No. 4,417,001 by replacing a certain amount of the reactive polyol component with a digestion product of PET. Polyester polyol compositions obtained from PET show partial crystallization, which lead to low storage stability of the mixtures. EP 0798 325 A1 reports polyhydroxy compounds from PET with increased chemical compatibility, which are suitable for polyurethane syntheses.
Non-reactive hot melts and/or pressure-sensitive adhesives have also been prepared using PET based polyester polyols. U.S. Pat. No. 5,008,366 and U.S. Pat. No. 3,993,629 disclose polyester based compositions using polyesters obtained from PET. An adhesive or coating composition that comprises a reaction product of PET polyester with a difunctional organic acid is reported in WO 95/21213.
EP 0 558 905 A1 describes the preparation of non-reactive, physically drying aqueous coating compositions from (optionally waste) PET. The polyethylene terephthalate is first reacted with an alcohol and then with carboxylic acids or their anhydrides to form low-molecular-weight polyesters, which can be further subjected to direct acrylic modification. Direct acrylic modification is typically conducted under conditions described in U.S. Pat. No. 4,735,995 and U.S. Pat. No. 4,873,281. The low-molecular-weight polyesters that may contain reactive double bonds are reacted in the presence of organic solvents and ethylenically unsaturated vinyl monomers after a free radical polymerization. Direct acrylic modification means in this case copolymerization with acrylic monomers and not functionalization of the polyester to obtain polymerizable acrylic ester monomers for radically curing acrylic systems.
Hence, it was a general object of the present invention to provide low-cost and low-odor (meth)acrylic ester binders which can be used as reactive components for reactive acrylic systems, particularly for adhesives, coatings, floorings, mortars, and casting compounds with enhanced mechanical properties, and thatxe2x80x94besides of fulfilling the technical requirements of acrylic systems for industrial applicationsxe2x80x94are, from the economical and ecological point of view, preferably superior to already commercially available (meth)acrylic ester monomers. Other objects of the present invention are a method for the production of respective binders and their use.
Now, in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the (meth)acrylic ester binders of the present invention are manifested by the features that they are mixtures of oligomers, said oligomers comprising those of the following formula
xe2x80x83Axe2x80x94[R1]nxe2x80x94Xxe2x80x94R2xe2x80x94Oxe2x80x94[COxe2x80x94R3xe2x80x94COxe2x80x94R4]mxe2x80x94Axe2x80x83xe2x80x83(I)
and/or
Axe2x80x94Oxe2x80x94R2xe2x80x94Xxe2x80x94[R1]nxe2x80x94[COxe2x80x94R3xe2x80x94COxe2x80x94R4]mxe2x80x94Axe2x80x83xe2x80x83(II)
wherein A represents 
CO is carbonyl group
R1 is a repetition unit of an aromatic polyester, in particular a repetition unit of PET and/or PEN,
R2 is a divalent radical selected from the group consisting of optionally substituted linear and branched C3-C20 alkylen, cycloalkylen and aralkylen radicals, optionally substituted di-, tri- or tetraalkylenether radicals, and optionally substituted heterocyclic radicals, preferably such radicals derived from the alcohols or aminoalcohols mentioned above, whereby said substituents are preferably selected from the group consisting of hydroxy groups, ester groups and alkyl group,
R3 is a divalent radical selected from the group consisting of optionally OH or COOH substituted linear and branched aliphatic and aromatic and araliphatic radicals, in particular radicals with 3 to 14 C atoms
R4=xe2x80x94Oxe2x80x94R2xe2x80x94Xxe2x80x94[R1]nxe2x80x94 or xe2x80x94[R1]n-Xxe2x80x94R2xe2x80x94Oxe2x80x94 or xe2x80x94Xxe2x80x94R2xe2x80x94Oxe2x80x94
R5 is hydrogen or methyl group
X is xe2x80x94Oxe2x80x94 or xe2x80x94NHxe2x80x94, and
n is 1 to 4, and
m is 0 to 3.
The binders of the invention may contain, besides oligomers of formula (I) and/or (II), at least one compound which is selected from the group comprising
Axe2x80x94Oxe2x80x94R2xe2x80x94Xxe2x80x94A
Axe2x80x94[R1]nxe2x80x94Oxe2x80x94R6 
Axe2x80x94OH
wherein A, R1 and R2 are as defined above
R6 is a linear or branched aliphatic or aromatic or araliphatic radical in particular a radical with 5 to 22 C-atoms.
A specific example of a non-modified binder molecule is shown below. 
Preferred binders are those that are obtainable from glycolyzed aromatic polyester such as polyethylene terephthalate (PET). These binders are suitable as reactive components for reactive acrylic systems, more specifically for adhesive, coating and flooring compositions with:
excellent mechanical properties, such as high tensile strength, high compressive strength and high scratch resistance,
good weather and chemical resistance.
The (meth)acrylic ester binders of the present invention can be prepared by reacting polyesters containing free OH-groups with acrylic and/or methacrylic acid. The OH-terminated polyesters preferably are products of the reaction of high-molecular-weight aromatic polyesters such as PET with aromatic or aliphatic polyols and/or aminoalcohols.
The binders of the present inventionxe2x80x94as they result from the reactionxe2x80x94are mixtures of molecules comprising PET fragments of generally 1 to 4 PET units, preferably 1 to 3 PET units.
The binders of the present invention can be present as such, or in compositions with other (meth)acrylates or ethylenically unsaturated vinyl monomers, or in formulations comprising at least one of the following further substances, namely catalysts, stabilizers, initiators, organic fillers, inorganic fillers, additives, toughening agents, adhesion promoters, defoaming agents, thickeners, thixotropic agents, plasticizers, wetting agents, wax compounds, cross-linking agents, inhibitors etc. Such additional substances are known to the skilled person and comprise e.g. as catalyst: N,N-dimethyl-p-toluidine, N-ethoxylated p-toluidine, N-alkylmorpholine; as inhibitor: hydrochinone, methylhydrochinon, t-butyl-p-cresol; as initiator: benzoylperoxide.